Hi Stan,
The motion you described sounds like a lake phenomenon referred to as a
seiche; such oscillations have been detected on much larger scales, in lakes
and even Puget Sound. Sounds like you were fortunate to have had a model
seiche right in your own backyard. I'm envious, I didn't notice any quake
effects at all.

http://www.pep-c.org/seiche/

Kevin Li
Ballard, USA
kdli at msn.com